1. Field of the Invention
The present invention relates to a semiconductor device, for example, a metal-insulator-semiconductor (MIS) field-effect transistor, a semiconductor device having a structure in which a dangling bonds of a semiconductor are terminated by an insulating metal silicide thin film formed on a semiconductor substrate, and a method of manufacturing the semiconductor device.
2. Description of the Related Art
There has been a demand for thinning of a gate insulating film for miniaturization of an ultra large scale integration (ULSI) device or reduction of power consumption. The gate insulating film has heretofore been thinned in order to increase a capacitance and thereby secure an electric charge amount induced in a channel of a field-effect transistor (FET). As a result, the thinning of an SiO2 film, which is a gate oxide film, has been propelled, and the film has almost reached a thickness largely below 1 nm at present.
In the SiO2 film, a gate leakage current has increased to such an extent that the power consumption cannot be suppressed because of standby power dissipation. For example, even when the SiO2 film has a film thickness of 0.8 nm, MOSFET normally operates, but the gate leakage current reaches 1 kA/cm2, and the problem in the power consumption has become remarkably pronounced.
From a viewpoint how the power consumption be reduced, it is effective to increase an actual film thickness. Therefore, attempts to secure the charge amount with a film thicker than the SiO2 film by the use of a high-dielectric-constant material, so-called high-K dielectrics have been actively studied. There are many oxides as high-dielectric-constant stable materials, and they have been developed as insulating films. At present, as an especially prospective material, there has been HfSiON or the like. However, when high-dielectric-constant oxide is directly formed into a film on an Si substrate, a low-dielectric-constant layer is generated in an interface in the process of film formation or a subsequent high-temperature process. There is a demand for a film thickness, as an equivalent SiO2 film thickness (EOT), of 0.5 nm or less in the future, so that the generation of the low-dielectric-constant layer can be said to be fatal.
Moreover, with regard to the interface, defect density has a very important meaning. That is, when there are defects in the interface, movements of electrons or holes are obstructed, and mobility is remarkably deteriorated. An interface between SiO2 and Si, which has been used for years, can be constituted in such a manner as to have very few interface defects. In this case, a density of interface trap (Dit) can be reduced to the order of 109 cm−2 eV−1. From this respect, it is considered that the interface defects can be reduced to some degree with an SiO2-based silicate material, and the material has been intensively developed. However, silicate is not sufficient in the sense of dielectric constant for the next generation, and a high-dielectric-constant oxide needs to be developed as the insulating film for the future. However, this represents a large problem. When a high-dielectric-constant oxide is directly formed into a film on an Si substrate, there is a problem that very many interface defects are generated in the interface. For example, when CeO2 is actually epitaxially grown on an Si (111) surface, Dit reaches 5×1013 cm−2 eV−1. Moreover, in consideration of a high-speed operation of an integrated circuit, it is an essential proposition to suppress Dit to that of an SiO2/Si interface or less (order of 109 cm−2 eV−1 or less).
On the other hand, many methods of epitaxially growing an insulating material on the Si substrate have been proposed (see, e.g., Jpn. Pat. Appln. KOKAI Publication No. 2002-100766). In view of a strong demand for inhibition of generation of the low-dielectric-constant layer and minimization of the interface defect density, in any of the epitaxially grown insulating film which have heretofore been proposed, both the prevention of the low-dielectric-constant layer from being generated and the suppression of the interface defect density to a degree of the density of the SiO2/Si interface cannot be achieved at the same time. On the other hand, a large number of proposals that amorphous oxide be grown on Si have been made, but there has not been any development to solve the problem and achieve both the inhibition of the generation of the low-dielectric-constant layer and the minimization of the interface defect density.
Furthermore, it has also been pointed out that when a high-dielectric-constant oxide is epitaxially grown directly on Si, electron barriers cannot be sufficiently high, and a tunnel current cannot be suppressed (e.g., McKee, Science vol. 293 p468 (2001), etc.).
Moreover, a technique of forming Sr, Ti metal or the like into a thick film while SiO2 remains on the Si substrate has been reported in the following two papers (Ishiwara, Japanese Journal of Applied Physics 30 p1415 (1991), Ishiwara, Japanese Journal of Applied Physics 33 p1472 (1994)). In this case, the low-dielectric-constant layer is generated in the vicinity of the interface, and strictly speaking, it is impossible to achieve an equivalent SiO2 film thickness of 0.5 nm or less, which is necessary for the gate insulating film.
For example, when a Ti metal is formed into a film, Ti silicate is generated in the interface. Moreover, Ti has a further problem. Since the band offset of the conduction band of dielectric (TiO2 or SrTiO3) made in this film formation process is very low, Ti has not originally been suitable for the gate insulating film. This also applies to the following series of documents, which are aimed principally at the epitaxial growth of oxide on Si, and do not present any solution to the interface characteristic or band offset, which are very important for the field-effect transistor.
As development obtained by expanding the above-described two documents, a technique for obtaining Sr silicate has been reported in which an initial SiO2 film is thinned, and an Sr film is formed on the SiO2 film while changing the Sr amount (Journal of Vacuum Science & Technology B18 p2139 (2000) (Motorola)). However, after a heat treatment, the dielectric constant of the interface is lowered as low as that of SiO2. Since resistance to oxidation is very low in this manner, it is impossible to achieve an equivalent SiO2 film thickness of 0.5 nm or less, which is necessary for the gate insulating film.
A technique in which an SrO film is directly formed using hydrogen termination has also been reported (Saiki, Applied Physics Letter 65 p3182 (1994)). Hydrogen comes off the interface during film formation, but it is substantially impossible to terminate the dangling bonds of the Si interface by SrO, which is a material having no polarity. Therefore, a large amount of interface defects are generated. In general, when a hydrogen-terminated surface is used, the termination breaks during the film formation, and an interface portion becomes active. Even when forming gas annealing (FGA) is performed, recovery after the interface completion is limited. Since the recovery is not completed, reactivity of the interface remains as much. Therefore, interface oxidation proceeds depending on the post-processing temperature, and the low-dielectric-constant layer is generated. Even when the high-dielectric-constant material is epitaxially grown, the interface defect density is large (Dit=5×1013 cm−2 eV−1). Even when the FGA is performed, a recovery is expected only in an order of magnitude. It is also seen that when the annealing is performed in oxygen, low-dielectric-constant silicate is generated in the interface as described above.
A research has also been reported in which one mono-layer (1 ML) of SrSi2 is formed into a film, and excessive metal Sr is further formed into a film, and thereafter oxygen is introduced (McKee, Science vol. 293 p468 (2001), McKee, 2002 IEDM Technical Digest p955 (2002)). When excessive Sr is introduced onto SrSi2, a silicide film thickness increases even at low temperature, the insulating property is broken, and the termination structure cannot be constituted. Additionally, since time or Sr amount is not sufficiently controlled, the silicide film thickness is distributed at random, and the interface is considerably roughened. On a metal silicide roughened in this manner, SrO is formed into a film. Usually, the silicide is metallic. Even when SrO is formed on silicide, a band gap does not open. This is because SrO does not generate excessive bonds as an oxide. Since the dangling bonds are left in metallic silicide, the resistance to oxidation remarkably deteriorates. Reflecting that the metal silicide having a random thickness is introduced as described above, there are many defects in the interface. Even when the forming gas annealing is performed, the defect density Dit remains in the order of 1011 cm−2 eV−1. In the development performed in this document, there is a point where silicide such as BaSi2 and SrSi2 is adopted as a substrate for epitaxially growing mainly SrTiO3 or the like on Si. In this case, it is most important to take over lattice information of the substrate into an oxide or the like grown in an upper part, and a small number of defects may exist in the interface. The defects of the interface in a case where oxides such as SrO, BaO, SrTiO3, BaTiO3 are epitaxially grown on silicide are about 1012 cm−2 eV−1 at the film formation time (improvement in an order of magnitude is seen by the annealing after the film formation). This value of 1012 cm−2 eV−1 is very small for the substrate for taking over a lattice constant at the time of the epitaxial growth on Si. On the other hand, this is an excessively large value as the defect of the Si/insulating film interface in a case where the gate insulating film is formed on Si to form the field-effect transistor.
Development similar to the above-described McKee's development has been reported as follows (Norton, Journal of Vacuum Science & Technology B20 p257 (2002)). However, initially formed SiSr2 silicide has a differing film thickness, depending on the location, and the interface is in a very active state because of this film thickness profile. Therefore, a silicate structure is generated at the time of Ba film formation in oxygen at a low temperature of 550° C. As compared with the above-described McKee, there is a large film thickness change in the silicide of this document, therefore activity of the interface is higher, and resistance to oxidation is low.
In a case where a high-dielectric-constant film including silicate is directly formed on the Si substrate for a purpose of preparing the field-effect transistor, the interface low-dielectric-constant layer is generated, thus it is difficult to form a gate insulating film having an equivalent SiO2 film thickness largely below 1 nm (thickness of 0.5 nm or less is a target at present, but there is a possibility that the thickness further decreases in the future). As an example, Zr silicate is generated with ZrO2, and SiO2 is generated with SrTiO3 in the interface. There has thus been a demand for enhancing the resistance to oxidation. Even in a case where the film is directly formed on the Si substrate, and the low-dielectric-constant layer cannot be formed, it is meaningless if the interface defect density is large.
Moreover, in the above-described Jpn. Pat. Appln. KOKAI Publication No. 2002-100766, a field-effect transistor has been described in which the Si substrate is terminated by silicide having a thickness of two mono-layers or less, such as La silicide, and further the insulating film is formed thereon. However, the silicide of this document is regarded as a metal, thus reactivity is very strong, and the silicide is not suitable for forming a thin film. This is caused by a trivalent material such as La, in which the bond on the La side is excessive, and Si dangling bonds are not terminated well. Once a silicate is formed, oxidation of substrate Si proceeds at random, and the interface with the upper-part thin film is roughened, because silicate itself has a low dielectric constant, and oxygen cannot be blocked by silicate. This causes a problem in the gate insulating film.
Therefore, there has been a demand for realization of a semiconductor device in which the dangling bonds of Si of the semiconductor substrate are terminated by an insulating metal silicide thin film formed on the semiconductor substrate containing Si as a main component, and a method of manufacturing the semiconductor device.